This invention relates to a surge voltage arrester, and more particularly, to a surge voltage arrester of a gas-filled discharged tube type.
At present, a surge voltage arrester of a gas-filled discharge tube type is widely accepted in order to protect, for example, a transmission line or electric implements connected thereto from an excessively high surge voltage. However, the above-mentioned conventional surge voltage arrester has the drawbacks that when a high surge voltage is impressed, a time delay sometimes takes place in initiating the discharge of said high surge voltage. Consequently, the surge voltage arrester unduly fails in its insulation ability, eventually resulting in an insulation failure, and ceasing to protect the above-mentioned implements from a damaging high surge of voltage.
To date, therefore, various proposals have been advanced to eliminate the above-mentioned undesirable malfunctions of the conventional surge voltage arrester. For instance, U.S. Pat. No. 3,588,576 sets forth a device intended to eliminate the time delay in the initiation of the discharge when a high surge of voltage is impressed upon the aforementioned elements requiring protection from a surge of voltage. Said patented device is characterized in that a conductive layer of lumped potential gradient is provided in the vicinity of the main electrodes adapted for the discharge of a surge voltage arrester. According to this prior art, the lumped potential gradient is concentrated in the vicinity of the main electrodes when a high surge voltage is impressed on said surge voltage arrester, thereby initiating the discharge across said main electrodes. The surge voltage arrester of a gas-filled discharged tube type described in the aforementioned United States patent comprises: hollow cylinder prepared from insulation material such as ceramic or glass; a pair of mutually facing main electrodes held in said hollow cylinder with their discharge planes set apart from each other; and narrow conductive layers formed in the ionization region of said interelectrode space which extend along the inner wall of said hollow cylinder in the axial direction thereof. The surge voltage arrester of the aforementioned United States patent is described as having the characteristics that when a high surge voltage is impressed on the surge voltage arrester, the electric lines of force are concentrated on an area defined between the end portion of said narrow conductive layer and the pair of the main mutually facing electrodes, thereby quickly ionizing gas held in the ionization region of the interelectrode space and consequently accelerating the discharge of said main electrodes.
However, the surge voltage arrester set forth in the previously mentioned U.S. Pat. No. 3,588,576 has the drawback that noticeable current discharge often takes place between the paired main electrodes used with the surge voltage arrester. The discharge leads to the sputtering of part of the main electrode-constituting material. Sputtered particles settle on the inner wall of the hollow cylinder surrounding the main electrodes producing a thin conductive layer which reduces the insulation property of said surge voltage arrester due to the bridging of the narrow conductive layers. The discharge-initiating voltage impressed across the main electrodes drop from a prescribed level due to the above-mentioned sputtering, thus rendering said discharge-initiating voltage unstable.
Another known surge voltage arrester proposed for the diminution of the drawbacks of the aforesaid U.S. Pat. No. 3,588,576 is the type described in the specification of the U.S. Pat. No. 4,056,753 allowed by the present patent applicant. The surge voltage arrester set forth in said U.S. Pat. No. 4,056,753 comprises: a hollow cylinder prepared from insulating material; a pair of mutually facing main electrodes fitted in an airtight fashion to both of the open ends of the hollow cylinder to define a closed chamber with their discharge planes set apart from each other; and two mutually facing conductive layers extending along the inner peripheral wall of the hollow cylinder in the circumferential direction at a prescribed interval from the main electrodes, said two conductive layers being respectively provided with a projection directed toward said mutually facing main electrodes. The surge voltage arrester proposed in said U.S. Pat. No. 4,056,753 in which the conductive layers and main electrodes are capacitively coupled can indeed eliminate the drawback of a drop in insulation resulting from the above-mentioned bridging of the conductive layers. However, the surge voltage arrester described in said U.S. Pat. No. 4,056,753 is still accompanied with the drawback that since the two conductive layers are respectively provided with a projection extending toward the mutually facing main electrodes, complexities arise in the manufacture of the subject surge voltage arrester. When the discharge of the main electrodes is initiated, a discharge is concentrated on said projections. Particles sputtered from the main electrodes collectively settle on said projections resulting in a decline in the discharge initiating voltage between the main electrodes, and variations in the operation of the subject surge voltage arrester. The conductive layers and main electrodes are capacitively coupled to a small extent through a gas sealed in the surge voltage arrester. Therefore, it is necessary for the realization of a capacitive coupling to reduce an interval between the conductive layer and main electrode, or to broaden the width of the conductive layer. Though this process can indeed improve the characteristic of the impulse sparkover the voltage, yet a limitation is imposed on the space between the conductive layer and main electrode when considering the insulation resistance of the subject surge voltage arrester itself, or when considering the discharge initiation voltage characteristics of said arrester when impressed with a different type of overvoltage from that of an impulse. Further an attempt to broaden the width of the conductive layer unavoidably leads to an increase in the steps in manufacturing a surge voltage arrester and, consequently, to a decline in the manufacturing efficiency.
In addition to the above-mentioned surge voltage arrester, U.S. Pat. No. 4,287,548 sets forth another surge voltage arrester comprising a gas-filled housing incorporating two main electrodes disposed opposite one another, said electrodes being supported in a gas tight fasion by the ends of a tubular insulating member having at least one coating of electrically conductive material extending over a portion of the interior length of said insulating member, having at least one electrically conductive surface on the exterior of said insulating member which at least partially overlaps said coating, and having a means adapted to establish an electric field between said coating and said surface.
However, the surge voltage arrester described in U.S. Pat. No. 4,287,548 with reference to FIGS. 1 to 6, and FIGS. 8 and 9 is accompanied with the drawback from a strip 3 constituted by a coating electrically conductive material is connected to the main electrodes. When, therefore, the discharge of a large current often takes place between the paired main electrodes, part of the material constituting the main electrode gives rise to sputtering by said discharge. The sputtered particles of said main electrode material settle on the inner wall of the hollow cylinder surrounding the main electrode, causing the discharge-initiating voltage between the main electrodes to fall below a prescribed level, or rendering said discharge-initiating voltage unstable. Further, FIG. 7 attached to said U.S. Pat. No. 4,287,548, illustrates a center ignition strip 3 which is overlapped at both ends by the electrically conductive coatings 2 connected to the main electrodes. The center ignition strip 3 is constituted by a narrower conductive layer whose ends extend toward the main electrodes, and are overlapped by the conductive coatings 2. When, therefore, a discharge is started between the main electrodes, the discharge is concentrated at the ends of said strip 3. As a result, the sputtered particles of the main electrode-constituting material collectively settle on the ends of the strip 3 resulting in a decline in the discharge-initiating voltage between the main electrodes, or in variations in the operation of the subject surge voltage arrester. Since the strip 3 is formed of a narrow conductive layer, the capacitive coupling of said strip 3 with the main electrodes in the insulation member is effected only be a small extent. To attain the prescribed capacitive coupling, therefore, an attempt should be made to reduce the interval between the strip 3 and the main electrodes held in the insulating member. Though improving the impulse discharge-initiating voltage characteristic, the above-mentioned attempt is accompanied with the drawback that the insulation resistance of the surge voltage arrester is reduced. And when impressed with an overvoltage different from that caused by the impulses voltage, the discharge-initiating voltage characteristic of said surge voltage arrester is deteriorated.